The use of electrodes for performing radiofrequency ablation on certain parts of a patient's body is well known. Conventional electrodes are typically elongated, cylindrical shafts with insulation over a portion of the shaft. Such electrodes typically have an exposed, conductive tip, which is used to contact body tissue in a region where a heat lesion or ablation zone is desired.
In conjunction with such conventional electrodes, in most applications, large area or plate type electrodes are also commonly used to serve as reference electrodes. Such reference electrodes are placed external to a patient's body and never heated, but merely serve as a return path for the radiofrequency (rf) current circuit. These reference electrodes typically have a greater surface area than the surface area of radiofrequency (rf) ablation electrodes.
As a result of their greater surface areas, such reference electrodes spread or dissipate the radiofrequency current over a wide area of the tissue and consequently, prevent concentrated heating at any one point. Such reference electrodes are deliberately configured to remain cool as a safety precaution, to avoid burning surface tissue on a patient's body. To operate both the radiofrequency (rf) ablation electrode and reference electrode, they are connected to a radiofrequency generator, which provides the recurring current and voltage to produce the heat ablation around the conductive tip of radiofrequency (rf) ablation electrode. Such systems and techniques are described in many articles, as for example, a research paper by Cosman, et al., entitled "Theoretical Aspects of Radiofrequency Lesions in the Dorsal Root Entry Zone," Neurosurgery, December 1984, Vol. 15, No. 6, page 945-950. There are several types of radiofrequency equipment available, as for example, the electrodes and generators from Radionics, Inc., located in Burlington, Mass. In the research paper by Cosman, et al., a generally thin cylindrical ablation electrode is inserted into the body, and heating is enabled near it. The reference electrode, which is typically an area electrode, is placed on the patient's skin. The reference electrode specifically is much larger in surface area (for example, 150 square centimeters) than the thin cylindrical ablation electrode so that no substantial heating occurs near the reference electrode. Any such heating at the area electrode would cause skin burns, which is contrary to the radiofrequency technique described in the paper by Cosman, et al., referenced above.
Cylindrical electrodes are also commonly used for some applications. Cylindrical electrodes are typically metal tubes of 1 to 3 millimeters in diameter and several centimeters in length.
Typically, concentration of heat is maximum near the exposed conductive tip of the cylindrical electrode, with it progressively decreasing as the distance from the exposed tip increases. The degree of heat distribution depends on the radiofrequency current density in the tissue and electrical and thermal conductivities of the tissue near the electrode. Further details are discussed in the research paper by Cosman, et al., referenced above. Cooled radiofrequency electrodes can deposit heat at greater distances from the point at which the electrodes are placed. Yet, temperature inhomogeneities or hot spots can develop near the radiofrequency electrode, and this can lead to dangerous and uncontrolled boiling, charring, sticking, explosive steam formation, and hemorrhaging. This limits the amount of power that can be deposited into the tissue, limiting therefore, the volume of coagulated tissue.
Different techniques for ablation of cancerous tumors in the liver, brain, and elsewhere by use of such cylindrical, tubular, radiofrequency electrodes introduced into or near the tumor site, are discussed in a research paper by Cosman, et al., referenced above, as well as a research paper by Goldberg, et al., entitled "Tissue Ablation With Radiofrequency: Effect of Probe Size, Gauge, Duration, and Temperature on Lesion Volume," Acad. Radio., 1995, Vol. 2, No. 5, Pages 399-404. Carefully targeting the tumor site with the electrode is sometimes necessary, which requires stereotactic methods or iterative imaging of the patient's body while placing the electrode within the patient's body. Imaging can be performed by using ultrasound, CT, MRI, X-ray, or other techniques. If a tumor exists in the soft tissue of a limb, torso, neck, etc., and is rather large, it is difficult to determine the exact location in the tumor where the radiofrequency electrodes should be planted for ablation purposes. Thus, using discrete, cylindrical, radiofrequency electrodes in such cases, whether interoperatively or percutaneously, is not simple, and typically requires considerable imaging and careful planning prior to performing a procedure. By way of example, for large tumors, in an internal organ such as the liver, it is almost impossible for a surgeon, without use of sophisticated equipment to assist with targeting the desired location, to determine the exact location and size of the tumors.
Open surgical resection of a large sector of the liver is done routinely to remove regions where cancerous tumors are believed to exist. Such a procedure is possible only after using imaging techniques to determine the exact locations where the cancerous tumors are believed to exist. Such operations are technically challenging, morbid, and dangerous, often resulting in fatalities. They require an expensive and time-consuming surgical procedure. For a person in frail health or with significant health problems, undergoing such major surgery can be prohibitive or lead to extended recovery periods, which are inconvenient and costly.
Another known form of electrosurgery is often referred to as bipolar electrocautery or bipolar coagulation. To perform this procedure, a surgeon typically uses bipolar forceps, which are similar to surgical forceps, except that each arm of the forceps is insulated from the other and connected to a high frequency power source. Such bipolar forceps and coagulators are available from Radionics, Inc., located in Burlington, Mass. Such forceps typically have very small tips, which are conductive and therefore, serve as electrodes, contacting small volumes of tissue between them. Such tips typically have an area of no more than 4 to 6 square millimeters or 0.04 to 0.06 square centimeters. The purpose of these devices is to coagulate small volumes of tissue between the tips when the forceps are applied to the tissue and high frequency current is passed between the tips and through the tissue.
A common application where such forceps may be used is for purposes of coagulating small blood vessels or to stop bleeding during surgery. Often, the tissue that is coagulated can boil and char because of the very focused heat, which is caused by the small area forceps tips and the high density of coagulating current running through the tissue between them. Such small area tips would not be adequate to coagulate larger volumes of tissue lying within an organ or limb. For example, common dimensions for a tumor in the liver are typically between 1 centimeter and 6 centimeters or more. A tumor, whose size exceeds 1 centimeter would be too large to coagulate by using small area tip bipolar forceps of the type described above. If such bipolar forceps are in fact used, only tissue volumes of less than one cubic centimeter could be coagulated. Hence, this makes such forceps impractical for coagulating large tissue volumes, especially those exceeding 1 cubic centimeter in size, which is often desirable for minimally invasive procedures involving heat coagulation of tumors within organ or limb tissues. Moreover, use of small area bipolar forceps often result in inhomogeneities in heat distributions and uncontrolled hot spots or charring of tissue. Such side effects make their use for larger tissue volumes impractical and unsafe.
A less invasive system or method of ablating large volumes of tissue having cancerous areas would be desirable. A method and system, which is minimally invasive in terms of penetrating large tissue volumes, either through intact skin or interoperatively, and which would avoid heat inhomogeneities and hot spots is also desirable.